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The UK's new oil rush may have ended before it even began. There are several billion barrels of oil under south-east England, according to a new report, but it may not be worth drilling for it.

The report, by the British Geological Survey, estimates that there are 2.20 to 8.57 billion barrels of oil trapped in the shale rocks of the Weald Basin, south of London. However, it concludes there are no significant gas deposits in the area.

Energy companies will have to resort to the controversial technology of fracking to get at the oil. But they may decide not to bother, says petroleum geologist Andrew Aplin of Durham University in the UK. That's because very little of the oil can be reached, even with fracking.

Inconvenient numbers........

"Looking at data from the US, the exploitable amount of oil from fracking is normally around 5 per cent of the total reserve," Aplin says. This suggests only 110 to 428 million barrels of oil could be extracted from south-east England.

Even that might be optimistic. The 5 per cent figure comes from areas rich in limestone. In clay areas, like the Weald, the figure is lower. What's more, the oil in the Weald comes from similar rocks to North Sea oil, which is heavy and viscous. If Weald oil is the same, extraction will be difficult.

So Aplin estimates that only 1 per cent of the total reserve in the Weald – 22 to 86 million barrels – can be extracted.

"Britain consumes about half a billion barrels of oil per year, so if only 1 per cent is extractable that would be about two months' consumption," says Aplin. "It doesn't seem like a very big prize to me."

US fracking also suffered a blow this week. New figures suggest California's Monterey Shale holds just 600 million barrels of oil, not 13.7 billion as thought.